Electronic tilt measuring system

ABSTRACT

The invention provides an electronic tilt measuring system for measuring the relative tilts of various platforms such as weapons on a ship. The basic system comprises an inclinometer (1) at each platform, a central control and display unit (2) and a computer (4). The output signals from the inclinometers (1) are passed to the central unit (2) and averaged over a synchronized time period to give an average inclination measurement with respect to the gravity vector over the measurement period. The data is then passed to the computer (4) where it is analysed. The computer (4) is programmed to calculate measured tilts and relative tilts and to produce graphical representations of the data.

The invention relates to means for measuring the relative tilt of anumber of platforms to a nominated datum plane, for example for weaponplatforms on board a floating ship.

For a ship weapon system to detect, locate, identify, engage and destroya target successfully, it is fundamental that the initial alignment ofall the weapon and sensor platforms is correct. Thus, a system isrequired that is capable of accurately measuring the relative tiltsbetween equipment platforms on a floating ship. Furthermore, since workis carried out through all the stages of a warship's life, byshipbuilders and dockyards, it is necessary to be able frequently tomeasure and correct relative tilts between equipment platforms withrespect to the ship's Master Level Datum. If a relative tilt existsbetween equipments, errors in elevation and training of the weaponsresult.

The tilt of weapon equipments is defined as the angle by which theirplanes of rotations are tilted relative to each other, or to the ship'sMaster Level Datum. This datum platform is usually sited near theneutral pitch and roll axes of the ship and parallel to the calculatedwater line. The magnitude of the tilt is defined as the maximuminclination of the equipment seating relative to the plane of the datumplatform and the direction in which this occurs with respect to theship's fore and aft line (e.g. 1.5 minutes of arc at a bearing of 45° toPort).

If a relative tilt exists between equipments, the errors resulting inthe weapon alignments vary depending on the elevation and bearing of theequipments. When the equipment is trained to the tilt relative bearing,the elevation error is a maximum and equal to the tilt angle whilst thetraining error is zero and, when the equipment is trained at rightangles to the tilt relative bearing, the elevation error is zero whilstthe training error is proportional to the tangent of the elevation angleof the equipment. As the equipment elevates this training errorincreases, equalling the tilt angle at an elevation of 45° and becomingmore pronounced at higher elevations. For bearings other than thosealong or normal to the tilt relative bearing, errors exist in bothelevation and training.

To achieve desirable system accuracies it is necessary to reduce therelative tilt angles between equipments to within prescribed limits,either by mechanical or computer software corrections. This particularlyapplies to equipments capable of high angles of elevation.

The conventional technique for measuring weapon platform tilts used bydockyards and shipbuilders requires the ship to be positioned in dock,breast-shored in a stable position, but still just waterborne, beforecommencing a tilt test. This is necessary to restrict the motion of theship to enable measurements to be taken using bubble type clinometers.The loading of the ship must be as near as possible to sea-goingconditions and any movement on board must be kept to a minimum. Tiltrings, (for platform adjustments) are set to zero if they are fitted.

A 6° bubble type clinometer is set initially parallel to the fore andaft line of the ship on a convenient rigid position on each equipmentunder test. Each clino is placed on a special 3° wedge to ensure thatall readings throughout the test are positive.

A master clino is set up on a portable turntable, which is sited in asuitable ship position where communications are available to all weaponplatform stations under test and which is levelled to the horizontalplane.

The portable turntable is trained from 0° to 360° in 10° steps, at thesame time as the equipment under test. For each 10° step the operator atthe master position first adjusts his clino until the bubble is central.He then presses a bell push which rings a bell at all remote stations,to enable the clino readers at the platform stations to adjust theirclinos until all bubbles are synchronised with the master. Any movementin the ship during this period necessitates a repeat of the exercise onthat bearing. When all positions report that their clinos are lined upto the master, readings are taken and recorded and the equipment trainedto a new bearing.

Throughout the test the master clino reading is subtracted from theequipment clino readings on identical bearings and a sine curve for eachequipment is produced, to indicate errors in individual readings orserious non-flatness in the roller path.

On completion of the trial the differences between the clino readingsfor each pair of supplementary bearings are plotted on a radial plot inthe direction in which the highest reading occurs. A best fit circle isthen drawn through these points, from which the magnitude and directionof the tilt can be measured. These measured tilts are then replotted ona further tilt diagram, to enable the relative tilts between equipmentsto be determined. Corrections are then made by adjustment of theequipment tilt rings, where fitted, and a further cardinal points tilttest is then carried out to determine the residual tilt, which is thenfed into the weapon system computer software.

This tilt test technique, whilst effective in achieving the end result,is both inefficient and expensive in the use of manpower and resources.It imposes a severe burden on the limited docking facilities availabledue to the requirement to position a ship in dock, waterborne in abreast shored condition, for a period of up to 5 days. Thesedifficulties highlight the need for a system capable of permittingweapon alignment procedures to be carried out on a ship floating inharbour. Such a system would enable tilt tests to be carried out as andwhen required, at a fraction of the cost of the conventional technique.

An alternative system used an electrolevel in place of the traditionalbubble level. An electrolevel was sited on each equipment and theoutputs of each electrolevel were differenced to provide measurements ofthe tilt of each equipment. However, the electrolevel system involvedbulky, heavy equipment, was difficult to use and tended to be inaccurateand unreliable.

The object of the invention is to provide a system for measuringrelative tilts between various platforms with respect to a nominateddatum when the platforms may be subjected to motion.

It is a particular object of the invention to provide a tilt measuringsystem capable of being used on a floating ship to measure and correctfor relative tilts of ship's weapon and sensor equipment on differentequipment platforms.

The invention provides a means for measuring relative tilt of one ormore platforms to a nominated datum platform, comprising at least oneinclinometer per platform and characterised in that it furthercomprises:

means to average each inclinometer output over a synchronised timeperiod; and

a computer programmed to analyse measured data, calculate tilt anglesand relative tilts and display the results.

Advantageously a central control position is provided, and means todisplay the averaged inclinometer outputs over synchronised time periodsas readings are preferably included at the central control position.

An inclinometer is a transducer used by the system to determine the tiltof platforms. "Inclinometer" is the name ascribed to a low range linearaccelerometer which can be used as a slope detector because it is veryresponsive to changes in gravitational acceleration. The preferred typefor use in the invention is a solid state, dc, closed loop, forcebalance tilt sensor. This type of sensor is suitable for the inventionsince it includes a flexure supported torque balance system, ruggedenough to withstand severe shock and vibration and still maintainexcellent accuracy. Preferably each incinometer and its associatedelectronics are enclosed within a sealed housing, permitting operationin high moisture and salt laden atmospheres without deterioration inperformance.

Advantageously each inclinometer has built-in temperature compensation,thus permitting its operation over a wide range of temperatures withouta significant effect on the accuracy.

Preferably there is only one inclinometer at each platform.

Preferably there is also an inclinometer included at the nominated datumplatform.

Inclinometers are primarily devices for the static measurement of tilt.Thus when used in a dynamic situation on a floating ship the output ofeach device is affected by all linear accelerations experienced alongits sensitive axis. The output at any instant in time is therefore thealgebraic sum of any static tilt with respect to gravity, plus theacceleration component due to ship's motion.

When two inclinometers are used differentially to determine the relativetilt between equipment platforms, as in a tilt test, measurement errorswill result because the linear accelerations caused by ship's motionsare not equal at all positions. Significant variations are seen when therecorded outputs from inclinometers mounted in different positions arecompared. The mechanical stiffness of the various parts of the shipaffects the frequency content of the responses. Thus the relativemagnitudes of the accelerations at the two positions are dependent ontheir frequency content as well as their respective heights above theroll centre of the ship.

The measurements are derived by averaging each inclinometer output overa synchronised time period. Preferably this is done electronically. Thepreferred means is a dual slope integrating analogue to digitalconverter. Conveniently an averaging period of between 10 and 20 secondsis used as this will produce repeatable display readings to a fewseconds of arc. The resultant display reading represents the averageinclination with respect to the gravity vector over the measurementperiod and the algebraic summation of any two positions represents therelative tilt measurement on a given bearing. The display reading ispreferably given as an angle.

The averaging of the outputs is applicable because the difference inacceleration effects between any two positions approximates to zero whenaveraged over a time period that is significantly greater than theperiod of ship's motion, due to a basic similarity of the inclinometeroutputs and their sinusoidal nature.

Preferably the means to display the readings at the central controlposition is a display unit, which can be termed the master display unitor MDU. The MDU houses a display, conveniently an L.E.D. or liquidcrystal display, for each inclinometer on which the averaged outputreading of each inclinometer is shown for a particular measurementposition. The displays are conveniently calibrated to display minutes ofarc to one decimal place (i.e. resolution to 6 seconds of arc). Thedisplays are conveniently seven segment displays. Preferably the MDUhouses electronic circuits to process the signals to provide an averagedsignal over a set time period, the time period being synchronised forall the inclinometers.

Preferably the system includes means to calibrate the displays tocompensate for temperature variations. The calibration meansadvantageously include one or more temperature-stable voltage sources.

The computer is programmed to calculate the tilt angle, relative to thedatum platform, of each platform. The program advantageously uses aniterative method to produce a "best fit" sine curve for the recordeddata on all bearings at which readings are taken for each equipment.Preferably the computer is also programmed to calculate the relativetilt of each platform with respect to any selected datum platform.

The nominated datum platform may be a special purpose ship's MasterLevel Datum or a selected one of the ship's equipment platforms or anyother specified datum.

A means of communication is preferably provided between the platformsand the central control unit. A built-in communications system with awire link running through the cables used to interconnect the MDU andthe remote inclinometers is preferably used. Alternatively a batterypowered intercom system may be used. In some cases, for example on boarda ship, it is possible to use already existing communications and inthese cases a communications system would not need to be included in thetilt test equipment.

Optionally the inclinometer at each platform can be connected to a localdisplay unit to display the reading of that inclinometer.

The invention will be referred to as an Electronic Tilt Measuring System(ETMS).

The invention further provides a method of measuring relative tilt ofone or more platforms to a nominated datum platform comprising the stepsof:

1) positioning at least one inclinometer on the or each platform and atthe nominated datum platform;

2) connecting each inclinometer to a central control unit;

3) training all the platforms to a common bearing;

4) reading the measured values from each inclinometer at the centralcontrol unit and averaging each of the measured values over asynchronised time period;

5) entering the reading into a computer;

6) training the platforms to another common bearing;

7) repeating steps 4) to 6) as many times as required;

8) analysing the readings by means of a computer program; and

9) displaying or reading the results.

The platforms may be trained to any convenient number of bearings.Conveniently this may be 36 with each platform moved in 10° steps. Asmaller number of bearings such as 12 may be sufficient to achieveacceptable results, though, as the time saving is minimal in takingfewer results, it is preferable to take more results as a greater numberof readings provides a more accurate picture of the tilt of theplatform.

The readings of the measured values of the inclinometers may be feddirectly to the computer via a databus. In this case it is not necessaryto have displays of the readings at the MDU though it may beadvantageous to include such displays to increase user confidence of thesystem and allow progress of the test to be monitored. Alternatively theresults may be displayed, read by an operator and keyed into thecomputer manually.

When the test is completed on all bearings the data is analysed by thecomputer. Preferably a "best fit" sine curve is displayed for eachplatform on a visual display unit controlled by the computer.

Preferably the computer then determines and displays the relative tiltof all the platforms relative to any one of the other platforms or thenominated datum platform.

Adjustments can be made to the separate platforms to reduce the relativetilts. Additionally or alternatively for a ship's weapon system theresulting tilts may be fed into the ship's weapon system computersoftware to enable corrections to be made in the operation of the systemto allow for relative tilts.

The invention will now be described by way of example with reference tothe drawings, of which:

FIG. 1 shows a block diagram of an Electronic Tilt Measuring System(ETMS);

FIG. 2 shows a block diagram of a single channel of the ETMS of FIG. 1in more detail;

FIGS. 3a and b show how tilt errors occur when the equipment is trainedon the tilt relative bearing and at right angles to the tilt relativebearing respectively;

FIG. 4 shows a typical sine plot of measurements of a weapon platform;and

FIG. 5 shows a typical relative plot of the relative tilts of threeweapon platorms to a datum platform.

FIG. 1 shows a schematic block diagram of an Electronic Tilt MeasuringSystem. The basic system comprises inclinometers 1a,b,c and d, a MasterDisplay Unit 2, interconnection cables 3a,b,c and d, and a computer 4.

A typical commercially available inclinometer 1 is an extremely accuratedevice which is capable of responding to changes in angle as small as0.1 second of arc and with a quoted linearity of 0.05% of full scale.The inclinometer 1 is a closed loop servo accelerometer which works onthe principle of a pendulous mass with a single degree of freedom, whichreacts to an input along its sensitive axis causing the mass to move. Aposition sensor detects this minute motion and develops an outputsignal, which is demodulated, amplified and applied as negative feedbackto an electrical torque generator (torquer) coupled to the mass. Thetorquer develops a torque proportional to the current applied to it,which just balances the torque attempting to move the pendulous mass asa result of the acceleration input, preventing further movement of themass. This current which produces the equal and opposite torque istherefore proportional to the product of moment of inertia (a constant)and acceleration. If this current is passed through a stable resistorthe voltage developed across the resistor is proportional to the appliedacceleration.

The inclinometer output is thus an analogue dc signal directlyproportional to the angle of tilt. The range of the device chosen isquoted as ±1°, for which the dc output approximates as ±5 volts, thoughit has a usable range of up to ±3°. Trials so far carried out howeverhave indicated that the range of ±1° is more than adequate forconducting a tilt test on a floating ship, as normal ship's verticalityis within ±30 minutes.

The normal operating mode of the inclinometers is for the staticmeasurement of tilt of a surface. When used on board a ship which issubjected to a continuous oscillatory motion due to sea action, theoutput is an angle which varies with time. Under these circumstances,the accuracy of the inclinometer output is dependent upon its dynamiccharacteristics. A typical frequency response of a device such as this,which behaves like a damped simple pendulum with a characteristicnatural frequency, is for both the magnitude and phase of the measuredoutput to vary as the frequency increases with respect to the baseinput.

The relative dynamic characteristics between all of the inclinometers isa potential source of inaccuracy but a spectral analysis of theinclinometer output in response to a sinusoidal and a step inputconfirmed the bandwidth of the inclinometers to be above 3 Hz, which wasconsidered to be well outside the maximum frequency of ship's motion of0.5 Hz to which the inclinometers are required to respond in the presentinvention. The sinusoidal and step base inputs were first applied in astatic position in the laboratory. When used on board ship, there is thefurther complication of the motion which will produce an accelerationcomponent in the inclinometer output. The relative responses of theinclinometers to this effect were studied by comparison of the analoguerecordings obtained from two inclinometers mounted adjacent to eachother on board a floating ship when the conditions were severe.Recordings were taken for comparison at a number of positions around theship and at various heights above the roll centre of the ship. Therecordings were virtually identical and hence it was confirmed that theinclinometer responses were similar.

The individual inclinometer frequency responses showed that up to 0.5Hz, the frequency of interest, the magnitude of their outputs isconstant, with a small phase lag difference between extremes. This phaselag would produce an error in differential display readings proportionalto the magnitude of ship's motion if instantaneous measurements weretaken at two or more positions but because an averaging technique isused any resulting errors are insignificant.

The Master Display Unit (MDU) 2 is the master control position fromwhich the tilt test is coordinated. The MDU 2 houses four liquid crystaldisplays 5a,b,c,d, one for each remote position inclinometer 1a,b,c,d,with an appropriate display drive circuit (not shown) for each display.The displays 5 are calibrated to show minutes of arc to one decimalplace (i.e. resolution to 6 seconds of arc).

FIG. 1 indicates a number of other features included at the MDU:

a) A Display Freeze switch 6--this allows simultaneous freezing of allthe displays in the system, with a separate lamp 7 to indicate thecurrent state;

b) An Integration lamp 8--this gives a visual display of when anintegration phase is taking place;

c) A Display Read lamp 9--this gives an indication of when the Displayreading can be taken on a given bearing;

d) A Reset switch 10--this resets the Display Read lamp 9 to Off;

e) A Display Test switch 11--this sets all the displays readings to-188.8, to check their correct functioning;

f) Communications input 12--this is the input socket for acommunications system (not shown) to all remote stations;

g) DC Supply switch 13--to switch on the power supplies (not shown).

h) A Calibration switch 14--this enables all the displays to becalibrated simultaneously. During calibration (switch positions +, -, 0)a fast update mode (over a period of 0.3 seconds) is used. A slow updatemode (over a period of 64 seconds) is used during a tilt test. Duringthis 64 second period there is a synchronised integration phase for alldisplays of 1000 samples, equal to 16 seconds, during which the averagedc level of the inclinometer output for each position is determined anddisplayed;

i) Test Power inputs 15--allow the calibration to be checked externally;and

j) Adjustment controls 16a,b,c,d--these enable the displays to be tunedto give the correct readings when they are calibrated.

Details of the foregoing are not given as these will be readilyapparent.

The output signals from the inclinometers 1a,b,c,d are passed from theMDU 2 to the microcomputer 4 via a datalink 17 and an interface 18. Themicrocomputer 4 is programmed to analyse the measured data. A completeset of data can be analysed by this means in a few minutes, immediatelyon completion of a tilt test, as opposed to a delay of several hoursusing conventional techniques.

The computer first calculates the measured tilt of each weapon platformby producing a "best fit" sine curve to the recorded data on allbearings using an iterative method. It produces an amplified graphicalrepresentation of the difference in amplitude between the recorded valueand the sine curve value for each bearing. It also produces the moretraditionally accepted radial diagrams.

Subsequently the computer calculates the relative tilt of each weaponplatform with respect to either the Master Level Datum platform or anyselected master platform.

It is desirable for the ETMS to be as autonomous as possible so as toreduce reliance on ship's equipment and staff. Thus a communicationslink to all positions is included in the system (but not shown here).

The accuracy of the system is dependent on the magnitude of themeasurement errors. These are dependent on the accuracy of the devicesbeing used, their temperature stability, calibration and the design ofthe circuits.

Temperature effects are potentially the greates source of error inelectronic circuits but careful design can eliminate most of theproblems. The inclinometers 1 have two potential temperature effects,i.e. Scale Temperature Coefficient at 0.02% per °C. and Null TemperatureCoefficient at 0.05% Full Scale per °C., but these should not besignificant in normal usage.

The MDU 2 can be extended for further inclinometer readings by a "plugin" extension unit 19, here shown with eight further channels. This unitincludes eight further displays 5e to 5l, similar to displays 5a to 5d,which display readings corresponding to inclinometers le to 1l.

In FIG. 2, a block diagram of a single channel of the ETMS of FIG. 1 isshown in more detail. The arrangement allows two different methods ofmeasurement. In the first, the inclinometer outputs are averaged by adual slope integral analogue to digital converter 20 and displayed onthe displays 5. The displayed readings can then be entered by hand intothe computer 4. In the second method, the outputs from the inclinometer1, on each bearing, are passed to the computer 4 via a datalink 17 andan interface 18 and are averaged using a computer sampling technique.The averaged results are then stored by the computer 4 ready forcalculation of the tilts.

Before commencing a tilt test the displays 5 are calibrated by means ofa calibration circuit 21, at their temperature of operation, to read±60.0 minutes of arc using a temperature stable voltage source 22. Thisis a very simple and yet accurate method of calibrating the system andhas the added advantage of checking the electronic circuits. The netresult is that the display temperature errors are effectivelyeliminated.

The output voltage from the temperature-stable source 22 is used tocalibrate the displays 5. It is derived from the inclinometer output for±1 degree angle at 22° C. If the inclinometer were then used at zero°C., its output would be in error by 16 seconds of arc (FullScale×0.02%× degree C. change), due to the Scale Factor TemperatureCoefficient. The magnitude of this error is within the specifiedaccuracy required from the system but could be corrected via thecomputer software if necessary.

The Null Temperature Coefficient (Full Scale×0.05%×degree C. change) isa measure of the shift in the dc output from the inclinometer at zerotilt due to temperature. Because of the measurement technique usedduring a tilt test, any dc offset such as this does not affect theaccuracy of the measurement, providing the temperature remainsreasonably constant throughout the test. Experience has shown thattemperature variations are small but even if this were not so anyresulting errors would be insignificant.

As shown, the magnitude of the temperature errors when based on the fullscale output of the inclinometers of ±1 degree angle, are small. Theseerrors, however, become insignificant when the tilt test technique isconsidered. Reciprocal bearings are taken and added algebraically toproduce a radial plot. Therefore, because both readings are in error dueto temperature, the errors effectively cancel each other out with onlythe resultant difference reading between them being in error and as thisis rarely greater than a few minutes of arc, the magnitude of anytemperature errors will be a few seconds of arc at the worst.

The accuracy of the component parts has been examined in laboratoryexperiments. These have demonstrated that the measurement accuracy of astatic single reading to be of the order of ±6 seconds of arc. Theinaccuracies due to most of the component parts of the system are toosmall for consideration.

The greatest sources of measurement errors are the inclinometers whichhave a quoted linearity of ±0.05%, which is approximately ±2 seconds ofarc for their full range output of ±1 degree and the LED displays whichhave a resolution of 1 digit or 6 seconds of arc.

For accurate measurement, the calibration of the instrument must beaccurate to start with. The static calibration of the four inclinometersof FIG. 1 was checked periodically between tilt tests. This checking iscarried out by a very simple yet accurate technique using a calibratedsine bar and wedges on a surface plate to tilt the inclinometer by anextremely accurate angle. Measurements have confirmed the repeatabilityof the inclinometer outputs and the reliability of the devices.

FIGS. 3a and b illustrate the principles involved in elevation andtraining tilt errors where the plane of an equipment roller path isrelated to the plane of the master level datum. FIG. 3a shows how, whenthe equipment is trained to the tilt relative bearing, the elevationerror β is a maximum and equal to the tilt angle θ whilst the trainingerror γ is zero. FIG. 3b shows how, when the equipment is trained atright angles to the tilt relative bearing, the elevation error β is zerowhilst the training error γ is proportional to the tangent of theelevation angle α of the equipment. As the equipment elevates thistraining error γ increases, equalling the tilt angle θ at an elevationof 45° and becoming more pronounced at higher elevations. For bearingsother than those along or normal to the tilt relative bearing, errorsexist in both elevation and training.

To achieve system accuracy, the relative tilt angles between equipmentsmust be reduced to within prescribed limits. This is particularlyapplicable to equipments capable of high angles of elevation because ofthe increase in the training error γ at high angles of elevation asshown in FIG. 3b. The corrections are achieved by mechanical or computersoftware controls.

The tilt test is carried out with the ship tied up alongside in harbour.There are no restrictions on weather conditions or general movement onboard ship.

The Master Display Unit 2, from which the test is controlled, ispositioned in any convenient office on board ship and the cables 3 arerun out to the remotely positioned inclinometers. Before commencing atest the equipment is switched on and the electronic circuits allowed towarm up for a short time as the equipment stabilises to the surroundingair temperature. The displays 5 and/or the computer 4 are thencalibrated to the reference source 21.

The tilt test commences when the Master position instructs allequipments to be moved to a common bearing. After all equipments haveconfirmed the bearing the inclinometer outputs are fed to the computer.When the computer has taken the readings the program indicates that thenext bearing can be set and the Master position instructs all equipmentsto train to the next bearing. The readings are then taken and theprocess is repeated for 36 bearings.

On completion of the trial, the data is analysed by the computer and abest fit sine curve for each equipment displayed. The relative tiltbetween all equipments is then determined with reference to a selectedMaster platform.

FIG. 4 shows a typical sine plot of the measured tilt calculated by thecomputer for a weapon equipment platform on board a ship. Each reading23 was plotted on a graph showing 0° to 360° along the x-axis 24. Thescale on the y-axis 25 depends on the maximum values measured. A "bestfit" sine curve 26 is produced by an iterative method and displayed onthe graph. The computer further produces an amplified graphicalrepresentation 27 (scale ±120 seconds of arc) of the difference inamplitude between the recorded value and the sine curve value for eachbearing. These give a graphical representation of the equipment tilts.The computer further processes this information to produce values of thetilt of each platform relative to a selected Master datum. Correctioncan then be applied mechanically or the information can be fed into theweapons system software so that corrections can be made within thesoftware control to allow for the relative tilts between all theequipments.

FIG. 5 shows a typical relative plot calculated by the computer from theinformation used to produce FIG. 4. Point A is the reference datum towhich the tilts of the weapon platforms are calculated. Each dashed ring28 represents 0.5 mins of tilt from the datum. Each point B, C and Dmark the maximum tilt of the respective weapon platforms. Thus B has amaximum tilt of 2.6 mins at a bearing of 103° to port, relative to thedatum platform A, C has a maximum tilt of 1.4 mins at 138° to port and Dhas a maximum tilt of 0.7 mins at 039° to port.

The overall saving in time and manpower with ETMS is considerable. Themeasurement and correction of weapon platform tilts can be completed ina day by a small team of people, whereas the current technique employs alarge workforce and takes 3 to 5 days, including docking, breast-shoringand undocking.

Because the technique of the invention averages motion effects duringthe integration period there are no limitations and restrictions on whenand where the system can be used, providing measurements are confined tothe linear range of the inclinometers. The repeatability of the readingsalso makes the system suitable for other possible applications onfloating ships where accurate alignment between remote positions isrequired e.g. to assist in aligning the horizontal axis of a theodoliteto the ship's master level datum before use on weapon alignment.

The current methods of conducting tilt tests are inefficient in the useof both manpower and resources. The ETMS offers an alternative which isquicker, simple to use, at least as accurate and represents aconsiderable saving in both dockyard and operational costs, at what is arelatively small initial cost per system.

Although the device has been described in relation to a four channelunit with an eight channel extension, it will be clear that any suitablenumber and arrangement of channels in a unit can be used.

It will be obvious that any required number of inclinometers can beincluded in the system.

The invention is not limited to the measuring of tilts of ship weaponsplatforms. It can also be used when setting up fixed weapon positions,for example on fighter aircraft, and for checking the positions of otherequipments. It can also be adapted for checking the alignment of remoteaxes, for example on long shafts, or for checking the axes of equipmentssuch as gyroscopes.

It is claimed:
 1. A means for measuring relative tilt of one or moreplatforms to a nominated datum platform, comprising at least oneinclinometer per platform and characterised in that it furthercomprises:a) means to average each inclinometer output over asynchronised time period; and b) a computer programmed to analysemeasured data, calculate tilt angles and relative tilts and display theresults.
 2. A means for measuring relative tilts according to claim 1characterised in that a central control position is provided to averageeach inclinometer output over the synchronised time period.
 3. A meansfor measuring relative tilts according to claim 2 characterised in thatmeans to display the averaged inclinometer outputs as readings areincluded at the central control position.
 4. A means for measuringrelative tilts according to claim 4 characterised in that the means todisplay the readings at the central control position is a master displayunit (MDU) which houses electronic circuits to process each signal andto provide an averaged signal over a set time period, the time periodbeing synchronised for all the inclinometers.
 5. A means for measuringrelative tilts according to claim 4 characterised in that the systemincludes means to calibrate the displays to compensate for temperaturevariations.
 6. A means for measuring relative tilts according to claim 1characterised in that the inclinometers are solid state, dc, closedloop, force balance tilt sensors.
 7. A means for measuring relativetilts according to claim 1 characterised in that each inclinometer hasbuilt-in temperature compensation.
 8. A means for measuring relativetilts according to claim 1 characterised in that there is aninclinometer included at the nominated datum platform.
 9. A means formeasuring relative tilts according to claim 1 characterised in that theaveraging of the inclinometer outputs is done by electronic means.
 10. Ameans for measuring relative tilts according to claim 9 characterised inthat the averaging means is a dual slope integrating analogue to digitalconverter.
 11. A means for measuring relative tilts according to claim10 characterised in that an averaging period of between 10 and 20seconds is used.
 12. A means for measuring relative tilts according toclaim 1 characterised in that the computer is programmed with aniterative method to produce a "best fit" sine curve for the recordeddata on all bearings at which readings are taken for each equipment. 13.A means for measuring relative tilts according to claim 12 characterisedin that the computer is also programmed to calculate the relative tiltof each platform with respect to any selected datum platform.
 14. Ameans for measuring relative tilts according to claim 2 characterised inthat a means of communication is provided between the platforms and thecentral control position.
 15. A means for measuring relative tiltsaccording to claim 1 characterised in that the inclinometer at eachplatform is connected to a local display unit to display the reading ofthat inclinometer.
 16. A means for measuring relative tilts according toclaim 1 characterised in that each inclinometer and its associatedelectronics are enclosed within a sealed housing.
 17. A method ofmeasuring relative tilt of one or more platforms to a nominated datumplatform comprising the steps of:1) positioning at least oneinclinometer on said one or more platforms and at the nominated datumplatform; 2) connecting each inclinometer to a central control position;3) training all the platforms to a common bearing; 4) reading themeasured values from each inclinometer at the central control positionand averaging each of the measured values over a synchronised timeperiod; 5) entering the readings into a computer; 6) training theplatforms to another common bearing; 7) repeating steps 4) to 6) as manytimes as required; 8) analysing the readings by means of a computerprogram; and 9) displaying or reading the results.
 18. A method ofmeasuring relative tilts according to claim 17 characterised in that theplatforms are trained to 36 bearings with each platform moved in 10°steps.
 19. A method of measuring relative tilts according to claim 17characterised in that a "best fit" sine curve is displayed for eachplatform on a visual display unit controlled by the computer.
 20. Amethod of measuring relative tilts according to claim 19 characterisedin that the computer then determines and displays the relative tilt ofall the platforms relative to any one of the other platforms or thenominated datum platform.
 21. A method of measuring relative tiltsaccording to claim 20 characterised in that the relative tilt resultsare used to determine the adjustments to be made to the separateplatforms to reduce the relative tilts.